The grain refinement of Al alloys has been extensively investigated for several decades both in industry and academia, not only through attempts at developing efficient grain refiners, but also with a view to achieve a better understanding of the grain refinement mechanism [1-7]. The addition of Al-Ti-B grain refiners (e.g. Al-5Ti-1B, wt. %) during the grain refinement of Al alloys has been widely investigated due to these compounds’ higher nucleation potency and wide potential for industrial applications. Various theories regarding the grain refinement mechanisms of Al-Ti-B refiners have been proposed [1-4]. Despite differences between all these theories, it is generally accepted that Ti has multiple roles within the Al melt. One role is to create an enriched Ti region leading to the formation of an Al3Ti monolayer necessary for the heterogeneous nucleation of Al on the stable boride substrates (TiB2) [5, 6]. The other role is to act as an effective growth restrictor factor . It is the combined effects of the enhancement in nucleation site numbers and of the growth restrictions that result in the formation of desirable, small uniform equiaxed Al grains. It is well accepted that Ti is more likely to segregate to the TiB2 / a-Al interface affecting its structure and thereby the constitutional undercooling at the solid-liquid interface [5, 6]. The presence of the Al3Ti monolayer is still fervently disputed [5-7]. To resolve this disagreement, an atomic scale experimental investigation on the heterogeneous nucleation interface between TiB2 and α-Al in conventional castings (e.g. in alloys containing < 0.15 wt. % Ti that have not been melt spun) would be required.
This paper employs atomic scale high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) to probe the heterogeneous nucleation interface of TiB2 in Al alloys, with a special focus on the partitioning of solute elements (Ti) to TiB2. A significant Ti partitioning to the whole surface of TiB2 was unambiguously observed in the commercial grain refiner (Al-5Ti-1B), as shown in Figure 1 and Figure 2. However, such prevalent Ti partitioning was not observed on the basal planes of TiB2 in Al-Si based alloys with high Si concentrations, which could be used to explain why Si poisoning occurs. In order to avoid / reduce the Si poisoning, CrB2 was added together with TiB2. In this case, a significant Cr partitioning to the whole surface of TiB2 was observed, which may minimize the lattice mismatch with Al and / or improve the stability of a Al3Ti monolayer on TiB2, as shown in Figure 3. This investigation provides a clearer picture about the heterogeneous nucleation interface between TiB2 and Al and further develops heterogeneous nucleation theory.
Jiehua Li gratefully acknowledges the financial support from the Major International (Regional) Joint Research Project (No. 51420105005) from China. The SuperSTEM Laboratory is the U.K. National Facility for Aberration-Corrected Scanning Transmission Electron Microscopy, supported by the Engineering and Physical Sciences Research Council (EPSRC).
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To cite this abstract:Jiehua Li, Fredrik Hage, Quentin Ramasse, Peter Schumacher; Probing the heterogeneous nucleation interface of TiB2 in Al alloys. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/probing-the-heterogeneous-nucleation-interface-of-tib2-in-al-alloys/. Accessed: May 26, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/probing-the-heterogeneous-nucleation-interface-of-tib2-in-al-alloys/